Flexographic printing involves inking a raised image on a flexible media which then comes in contact with the print substrate, such as paper or plastic. The ink from the raised image onto the print substrate. The flexible plate is made of a rubbery material which has a somewhat pliant nature, the extent of which depends on the smoothness and fragility of the substrate. Contrary to other print processes such as offset lithography and gravure where high pressure is used during ink transfer, it is generally desirable to have a minimum of pressure between the raised inked image on the plate and the substrate. Too little pressure and no ink transfer or very uneven ink transfer will occur. Too much pressure and the pliant surface of the flexible plate will be squashed into the substrate blurring the image edges resulting in poor print quality.
Because of the requirement to work at minimal pressure for optimum quality, the distance between the plate surface and the substrate must be the same over the entire surface. While this depends on the uniformity of the press cylinder on which the plate is mounted, it also depends on the thickness uniformity of the flexible plate.
Methods for flexographic plate imaging by laser ablation with plates mounted on cylindrical drums is well known. The main application is in gravure and flexography printing industries where lasers are used to create ink carrying pits so the drums are able to transfer images directly or indirectly onto paper or polymer films. The techniques used are well developed and a wide range of lasers are used to create pits directly in metal drums or in drums coated with ceramic, rubber, or polymer layers. U.S. Pat. No. 5,327,167 (Pollard) describe a machine for ablating pits of variable density on the surface of a printing drum.
The lasers used are usually focused to spots on the drum surface with a diameter of 10 to 100 μm. Pits may be created by direct laser ablation or by ablation of a thin mask followed by chemical etching.
The drum or sleeve eccentricity as well as media thickness variations impact laser focusing and may lead to unacceptable defocusing. To eliminate this problem an autofocus system is required. The autofocus system described in WO 2009/115785 provides for measuring a distance to media just before imaging (engraving) and for subsequent corrections of the imaging lens position according to comparing of the resulted measurement with required focus distance.
FIG. 1 shows an example of an apparatus that can be used to ensure that the image created by the projection system remains in focus on the drum surface even if the drum varies in diameter, is not perfectly circular, or is mounted eccentrically on its axle. A cylindrical drum 104 is mounted on an axis about which it rotates. Cylindrical drum 104 is shown rotating in a clockwise direction 108. A laser beam 128, passes through lens 132 and creates an image on the surface of the cylindrical drum 104. Lens 132 and additional imaging optics components are attached to a carriage on a servo motor driven stage 124, to allow the optical components to move together along the Z direction 120 which is parallel to the projection system optical axis and perpendicular to the drum surface.
The stage that supports the carriage holding both the lens 132 mask is itself attached to second carriage. This second carriage is driven by a second servo motor driven stage which has a direction of motion parallel to the drum axis. This second stage, which is not shown in FIG. 1, has the function of moving the projection system and associated homogenizer along the length of the cylindrical drum 104. An optical sensor unit 112 is attached to the second carriage so that it moves down the length of the cylindrical drum 104 with the projection optics.
The sensor 112 is mounted such that it measures the relative distance from the sensor 112 to the drum surface at a position on the surface that is about to be exposed to laser pulses. The distance data generated by the sensor 112 is processed by controller 116 and used to drive the servo motor on the projection system stage in order to maintain the distance from the lens 136 to the drum surface at the process point constant so that the imaging is always in focus. For this application, the cylindrical drum 104 is expected to be made with some precision so that as it rotates and the optical projection system traverses the full length of the cylindrical drum 104, variations in the surface location and hence movement of the projection optics in the Z direction are expected to be small.
The system as described above is limited to only one layer engraving. This is due to the fact that after the first engraving layer is completed, it limits the performance of an autofocus system for engraving of subsequent layers, both from the point of view of distance to media sensing as well as from dynamics of lens movement.
A first of this invention is to provide an autofocus system which is capable of maintaining constant focus distance between the drum or sleeve media surface and imaging lens in one and more than one engraving cycles.
A computer that supports engraving by multiple laser channels may be heavily loaded due to the need to process 3D image engraving data. In this case, additional autofocus tasks may affect the functionality by lowering the calculation speed. Hence a second purpose of this invention is to reduce the load of the machine computer while 3D image engraving process is performed.